The present invention relates to a method and a device for compensating for image motion in an aerial camera in which the slot of a focal-plane shutter travels transverse to the direction of film transport and in which film transport is parallel to the direction of flight.
Devices to compensate for image motion, generally abbreviated "FMC" ("forward-motion compensation") or "IMC" ("image-motion compensation"), are included in aerial cameras of practically all types. They compensate by controlling relative movement between lens and film, to accord with the image-motion speed (V.sub.B) of the photographed object in the film plane, which speed is described by the equation ##EQU1## In this equation: V.sub.F is the speed of flight
f is the focal length of the camera PA1 h is the flight altitude or ground clearance
However, the image-motion speed V.sub.B is constant over the entire image field only if the optical axis of the camera lens is perpendicular to the terrain scanned by the flight. If this prerequisite is not satisfied, for instance because the airplane has tilted out of its horizontal position to the extent of angle .omega. during the exposure, or because the camera (i.e., its optical axis) has been installed at a lateral inclination angle .beta. to the vertical, then the image-motion speed V.sub.B changes within the image field as a function of the lateral extent of the field of view, viz. the field angle .alpha..
The foregoing can be noted qualitatively from the showing in FIG. 1 of the drawings herein. In the circumstance of lateral tilt (from the vertical) of the axis of the lens 4 of an aerial camera in an airplane 3, the object plane 2 and the image plane 1 are no longer parallel to each other and, accordingly, the camera-to-subject distance and the image scale are dependent on the instantaneous field angle .alpha.. The general expression for image-motion speed V.sub.B, taking roll and tilt into consideration, is given by the equation: ##EQU2## which .beta.'=.omega.+.beta..
It is clear that motion blur can occur in the picture if the influence of the additional parameters .beta.' and .alpha. is not taken into account when designing a device for image-motion compensation (FMC).
In so-called panoramic cameras, it is known to control the FMC motion not merely as a function of the ratio V.sub.F /h but, furthermore, as a function of the swing angle .gamma. of a prism which is arranged in front of the camera lens and which cyclically scans the terrain.
FMC systems for panoramic cameras are illustratively described in the Manual of Photogrammetry, Third Edition (1966), pages 145-146, as well as in West German published patent application OS No. 1,772,429. However, in such cameras, film transport in the image plane is not parallel to the direction of flight but is, rather, transverse to the direction of flight. Accordingly, in order to compensate for image motion, the lens must itself be moved transverse to its axis, thus requiring precise guidance of this camera part, which is of relatively great weight. Furthermore, panoramic cameras are not directly comparable to conventional cameras, since the panoramic camera always operates on the imaging side with a field angle .alpha. of 0, so that the term sin .beta.'.multidot.tan .alpha. in equation (2) plays no role.
Still further, panoramic cameras in general have the disadvantage that the film strip must be cut and spliced for stereoscopic evaluation, which means a considerable loss of time in the case of reconnaissance cameras. And the evaluation is made difficult by the fact that the involved use of rotating prisms precludes a linearly true imaging of the object.
On the other hand, in aerial photographic systems known as strip cameras (also known as "continuous strip cameras", or "Sonne cameras"), the film travels parallel to the flight direction. Film-transport speed is maintained at a value proportional to V.sub.F /h, while the film is continuously exposed in a narrow strip-shaped region which extends transversely to the flight direction. Such camera systems, as described, for example, in U.S. Pat. Nos. 2,413,349 and 3,163,098, are, however, suitable only for vertically downward observation. There is therefore no problem of adapting film speed to image motion, as a function of the angle of roll or tilt.
It is further known that continuous-strip cameras do not provide a picture that is capable of stereoscopic evaluation, since there is no overlap. Still further, aircraft roll produces strong distortions of the picture transverse to the direction of film transport.
British Pat. No, 885,690 describes an aerial-camera system with central-shutter cameras which are swingable or swung with respect to the vertical. Image-motion compensation is effected by moving the entire camera during the exposure, in accordance with a signal proportional to cos .beta.'.multidot.V.sub.F /h. Fine FMC correction as a function of field angle .alpha. is not effected in this camera.
In the camera known as KRb6/24, film-transport is in the flight direction, and a slotted shutter continuously travels transverse to the direction of film transport. To compensate for image motion in this camera, film-transport speed is controlled in accordance with Equation (1) above, modified by a superposed correctional movement which is dependent on field angle. The correction on the film drive is via a differential interposed between the film-drive roll and the drive motor, which rotates proportionally to V/h. The magnitude of this correction movement is adjustable, via lever mechanism, to the pre-set tilt angle .beta. and is produced by an eccentric which is so coupled to the shutter drive as to produce a movement which approximates the speed profile of image motion during the exposure.
A first disadvantage of this technique of image-motion compensation is the necessarily considerable expense for precision mechanical parts such as differential, lever transmission, eccentric, etc. Moreover, the described fine correction for image-motion compensation is not satisfactory, since errors occur as a result of gear play and torsion of the transmission shafts. A further source of error lies in the lack of constancy of the speed of the continuously running shutter; this affects image-motion compensation, since the corrective motion of the eccentric is derived from shutter speed.
Furthermore, use of the eccentric restricts camera use to a single speed profile which is adapted to the involved camera lens. A change in the lens configuration, as for example, replacement of a single lens by a so-called multi-lens arrangement consisting of a plurality of lenses of different axial inclination associated with the same shutter, requires the provision of a completely different speed profile. The lever transmission permits only the pre-establishment of a constant tilt angle .beta. . And it is not for this known mechanical correction mechanism readily possible to take into account the continuously varying roll angle .omega. of the aircraft.
Finally, continuously adjustable frame rates cannot be obtained with a continuously running shutter; such frame rates are possible only with a shutter characterized by start-stop action.